1. Field of the Invention
The invention relates to a displacement detection device, a vehicle steering system, and a motor.
2. Discussion of Background
Conventionally, various displacement detection devices that include a displacement detection sensor have been proposed. U.S. Pat. No. 5,422,568 A1 describes that a Hall element is provided as a displacement detection sensor and a change in magnetic flux density, which occurs as a detection target (wheel) rotates, is detected as a displacement by the Hall element.
The Hall element is electrically connected to a power supply that supplies electric power to the Hall element for its operation in a displacement detection device that includes a plurality of Hall elements in order to, for example, improve redundancy, the following measures against a short-circuit fault, of one of the Hall elements are taken.
According to a first measure, the Hall elements are provided with individual power supplies. With this configuration, even if one of the. Hall elements is short-circuited, the remaining Hall elements are still operable. That is, as shown in FIG. 5, power circuits PA to PC are connected to a plurality of Hall elements HA to HC, respectively. In this case, for example, even if the Hall element HA is short-circuited, it is still possible to supply electric power from the power circuits PB, PC to the remaining Hall elements HB, HC. Therefore, it is possible to continue displacement detection with the use of the Hall elements HB, HC.
According to a second measure, there is employed a single power supply that has a current supplying capability which is high enough not to cause a voltage drop even if one of the Hall elements is short-circuited. That is, as shown in FIG. 6, a plurality of parallel-connected Hall elements HA to HC are connected to a single power circuit P, and limiting resistors RA to RC are connected in series with the Hall elements HA to HC, respectively. In this case, for example, even if the Hall element HA is short-circuited, when the power circuit P has a current supplying capability that is high enough to supply a current that is higher than or equal to a value of current flowing through the Hall element HA, it is possible to continue displacement detection with the use of the remaining Hall elements HB, HC.
However, according to the first measure shown in FIG. 5, multiple power circuits (PA to PC) are required, which increases the circuit size as a whole device. In addition, the power circuits are individually connected to the Hall elements (HA to HC). Therefore, there occur variations in voltages applied to the Hall elements, which may cause a detection error.
According to the second measure shown in FIG. 6, in order to increase the current supplying capability of the power circuit (P), the size of the power circuit is set large. In addition, if the resistance values of the limiting resistors (RA to RC) are increased in order to reduce a value (current limiting value) of current flowing through a short-circuited Hall element, voltages applied to the Hall elements decrease by an amount, of an increase in the amount of voltage drop in the limiting resistors. This reduces a dynamic range of displacement detection.
In addition, Japanese Patent Application Publication No. 9-318305 (JP 9-318305 A) describes a position detection device in which two Hall elements, which serve as displacement detection sensors, and a photo-interrupter are electrically connected to a single power supply. However, the photo-interrupter is used to correct, as an additional measure, a detection result obtained by the Hall element, having a large detection error. Therefore, the photo-interrupter is not used to interrupt supply of power.